1. Field of the Invention
The present invention relates to wireless telecommunications generally, and more particularly to the application of a smart antenna to PN code acquisition in a code division multiple access (CDMA) wireless communications system.
2. Description of the Related Art
In third generation (3G) wireless communications systems, particularly wide band (W)-CDMA systems such as those described in the 3rd Generation Partnership Project, xe2x80x9cPhysical Channels and Mapping of Transport Channels onto Physical Channels (FDD),xe2x80x9d 3GPP Technical Specification, TS25.211, v3.2.0, March 2000; 3rd Generation Partnership Project, xe2x80x9cSpreading and Modulation (FDD),xe2x80x9d 3GPP Technical Specification, TS25.213, v3.2.0, March 2000; and 3rd Generation Partnership Project, xe2x80x9cFDD: Physical Layer Procedures,xe2x80x9d 3GPP Technical Specification, TS25.214, v3.2.0, March 2000 (collectively xe2x80x9c3GPPxe2x80x9d), or in the CDMA2000 standard in TIA, Interim VandV Text for CDMA2000 Physical Layer (Revision 8.3), Mar. 16, 1999 (xe2x80x9cTIAxe2x80x9d), an option is to employ smart antenna technology in the base station. A smart antenna can suppress interfering signals from different direction of arrival angles (DOAs) from the desired users by using spatial diversity. Smart antenna technologies attract much attention these days as they support more users with a high quality of service and high data rates, up to 1.92 mega bits per second (Mbps). Examples of smart antennas and system architectures employing smart antennas may be found in commonly-owned co-pending U.S. application Ser. No. 09/610,470, filed Jul. 5, 2000, entitled xe2x80x9cSmart Antenna with Adaptive Convergence Parameter;xe2x80x9d Ser. No. 09/661,155, filed Sep. 13, 2000, entitled xe2x80x9cSmart Antenna with No Phase Calibration for CDMA Reverse Link;xe2x80x9d and Ser. No. 09/669,633, filed Sep. 26, 2000, entitled xe2x80x9cNew Cellular Architecture.xe2x80x9d The contents of all of these applications are hereby incorporated by reference herein.
Despite the interest in smart antenna technology generally, little attention has been paid to pseudonoise (PN) code acquisition in CDMA systems that employ a smart antenna at a base station. As used herein, PN code acquisition refers to a portion of a process referred to in the art as synchronization. Synchronization is generally regarded as encompassing two processes: PN code acquisition (in which a phase error for a known PN code is resolved to within a specified boundaryxe2x80x94that is, a coarse PN phase code error correction), and PN code tracking, in which fine differences in PN phase code errors are detected and corrected. Thus, PN code acquisition, despite its misleading moniker, refers to a coarse correction for a PN phase error between a receiver and a transmitter (e.g., a base station and a mobile unit, or vice-versa), and does not refer to a process by which a PN code itself (as opposed to a PN code phase error) is detected.
PN code acquisition may be difficult when the smart antenna weight vector does not correspond to the desired signal""s DOA (because the smart antenna will suppress signals from other DOAs). Because of this potential problem, existing systems use only one (omnidirectional) element output out of the M array elements for PN acquisition purposes. See F. Adachi, M. Sawahashi, and H. Suda, xe2x80x9cWideband DS-CDMA for Next-Generation Mobile Communications Systems,xe2x80x9d IEEE Communications Magazine, pp. 56-69, September 1998 (xe2x80x9cAdachi et al.xe2x80x9d). Thus, the benefit of a smart antenna (e.g., the potential gain of the smart antenna) has not been used for PN code acquisition. This causes another problem as the mobile units in the system may transmit signals with low power because of the expected high smart antenna gain at a base station, with the result that the received signal-to-interference-plus-noise-ratio (SINR) at a base station may not be sufficient for PN code acquisition when only one element is employed.
The present invention is an efficient PN code acquisition scheme which employs multiple elements of an antenna array and an adaptive threshold. The invention is particularly useful for CDMA wireless communications, especially for Direct Sequence (DS-)CDMA wireless communications. The basic structure of preferred embodiments is the combination of a conventional PN correlation searcher, an adaptive beamformer, and an adaptive threshold setting circuit. During each observation interval, which consists of multiple chips, the adaptive beamformer adaptively updates the weight vector for the smart antenna elements (as used herein, a smart antenna element refers to a single antenna, such as an omnidirectional antenna, in an array of antennas that collectively form the smart antenna) using the accumulated received signal despread with trial PN code phase error as input, preferably at the chip rate. The adaptive beam former may use any one of a number of algorithms for this purpose. In a preferred embodiment, a normalized least mean square algorithm is used. A spatially correlated signal is then formed by weighting an accumulated value of the signal received by each antenna in the array over the observation period with the corresponding final weight of smart antenna weight vector as calculated by the adaptive beamformer algorithm. This spatially correlated signal is then compared to a threshold to determine whether PN code acquisition has occurred. If the threshold is exceeded, PN code acquisition is declared. Otherwise, a new trial PN code phase error is selected and the process is repeated.
In conventional serial search algorithms, the above-mentioned threshold for PN code acquisition has been fixed and can be calculated from a given false alarm probability Pf and a given bit-energy-to-interference power spectral density ratio Eb/Io. But in a real mobile fading environment, Eb/Io often varies. In preferred embodiments, an adaptive threshold setting algorithm is employed to adapt to varying environment for efficient PN code acquisition. An adaptive threshold setting algorithm has been analyzed for a receiver with a single antenna element, in Kwonhue Choi, Kyungwhoon Cheun, and Kwanggeog Lee, xe2x80x9cAdaptive PN code Acquisition Using Instantaneous Power-Scaled Detection Threshold Under Rayleigh Fading and Gaussian Pulsed Jamming,xe2x80x9d The 4th CDMA International Conference, Proceedings Vol. II pp. 162-169, Seoul, Korea, Sep. 8-11, 1999 (xe2x80x9cChoi et al.xe2x80x9d) , the contents of which are hereby incorporated herein by reference. The present invention develops an adaptive threshold setting algorithm for a receiver with multiple array elements. The adaptive threshold setting circuit is actually an average power estimator in preferred embodiments. While the adaptive beamformer updates the weight vector w(i) adaptively, the power estimator estimates the instantaneous received signal and interference power prior to PN code despreading for each observation interval of NTc seconds. The average estimated power is then employed to scale a fixed reference threshold to create the above-mentioned adaptive threshold used to determine whether or not PN code acquisition has been achieved. The PN code acquisition time with the proposed PN code acquisition scheme with M=5 elements, by way of example, can be 210% shorter at a given SINR than the PN code acquisition time for a system, such as the system described in Adachi et al., in which only a single element is used for PN code acquisition.
In one aspect of the present invention, the PN code acquisition system may be applied to a base station, wherein the antennas are in the base station. According to another aspect of the present invention, the system is applied to a mobile station, wherein the smart antennas are in the mobile station.